This invention relates to printing systems, and in particular to printing systems involving projection of ink from a nozzle point to a recording medium.
Prior art devices for recording with liquid ink may be categorized into those involving continuous or sporadic contact between a stylus and a recording medium, and those involving projection of ink onto a recording surface.
The latter devices, known as "ink spitters" or "ink jets", may be further classified as to the manner in which the flow of ink is regulated, and as to the method by which the ink is targeted onto the recording medium. Ink flow is generally regulated through electrical means. By contrast, a variety of techniques have been utilized in directing the ink stream. One approach has been to apply a charge to the ink drops, and to employ electrical fields to deflect the charged drops by an amplitude corresponding to an applied potential difference. Another method utilizes mechanical means of placing the ink drops, by inducing oscillations of the ink jet nozzle transverse to the axis of the nozzle.
The oscillating nozzle technique, as disclosed, for example, by Carl Hertz in U.S. Pat. No. 3,737,914, uses a recording sheet which moves continuously in a direction substantially transverse both to the axis of the nozzle and the axis of oscillation. The result is that the ink jet traces a sinusoidal scanning pattern, and the image may be controlled by such factors as the spread of the ink drops as they impinge on the recording surface, whether the ink jet is on or off, and the frequency and amplitude of oscillation.
Because the aerodynamic properties of the projected ink stream create distortions from the pattern described by the nozzle tip, these devices typically project ink either on the up or down stroke. Furthermore, the segments of the sinusoidal pattern near the peaks are customarily omitted. These techniques rely on the reasonable approximation that the targets of all ink drops thus projected will be aerodynamically shifted by an equal displacement. Customarily, the ink stream is broken up into a line of discrete drops, which trace a pattern of dots on the recording medium.
The parameters of the frequency and amplitude of oscillation are subject to physical limits defined by the device employed to induce nozzle oscillation. One such device is comprised of a galvanometer attached near the tip of a glass capillary tube, with the tip bent at a right angle. The periodic torsion induced by an AC current through the galvanometer windings causes the nozzle tip to oscillate. A range of frequencies may be obtained through this device, typically with a 1 to 2 KHZ upper limit, and no lower limit. This broad band device involves serious control problems, however, in that there is a phase lag at high frequencies between the actual location of the galvanometer and that perceived by a control mechanism, and this phase lag varies from frequency to frequency, making this device quite difficult to calibrate.
An alternative approach which avoids the drawback achieves the desired oscillation by means of a vibrating metal reed. The reed carries a capillary tube from which the ink is projected, with the oscillations of the device confined to the resonant frequency of the reed. This avoids the phase lag calibration problem. There is, however, a commensurate inability to continuously vary the frequency of the scan, resulting in difficulties in providing variable rate printing with this device. A factor which must be considered in this regard is the range of intended applications for the ink jet printing system. For applications such as production lines coding, a lower degree of image control will suffice. In this use, for example, some variance in the size of alphanumeric characters is permitted, and a system which gives approximate frequency control would meet the requirements.
Accordingly, it is an object of this invention to employ an ink jet printing apparatus of the oscillating nozzle type which may be accurately controlled. Another object of the invention is to avoid phase lags of unknown magnitude in the oscillating mechanism. A related object of the invention is to utilize for this purpose a vibrating reed which may be easily calibrated to adjust for phase shifts.
A further object of the invention is to employ a vibrating reed ink jet system which is capable of variable frequency printing. Yet another object is to use a method for this purpose which will meet the tolerances required for production line coding.